1. Field of the Invention
The present invention relates to a polarization mode dispersion compensation apparatus in an optical transmission system. More particularly, the present invention relates to a polarization mode dispersion compensation apparatus using a photonic crystal structure.
2. Description of the Prior Art
Polarization mode dispersion (PMD) occurring due to birefringence (i.e., double refraction) changes certain polarization states of optical signals traveling along an optical fiber, and induces a differential group delay time between two polarization states of the optical signals during the travel of the optical signals through the optical fiber.
Such a group delay coefficient is about 0.1 ps/km in a case of low-PMD optical fibers that have been recently mass-produced, and reaches as much as a few ps/km in a case of single-mode optical fibers that have been more conventionally produced. Further, the group delay time may be more than 20 ps in a case of a long-distance fiber optical connection such as a 100 km ground transmission system using the single-mode optical fiber, and may be more than 10 ps even in a case of a transoceanic connection using recent low-PMD optical fibers.
The phenomenon of group delay time generates a problem in signal transmissions in optical communication systems of more than 10 Gbps. However, it is well known that the group delay time occurring in a specific transmission fiber is not constant with time. In fact, the group delay time may vary from time to time with physical environments, such as temperature, pressure, and the like.
Several conventional methods for compensating for the polarization mode dispersion in such optical fibers have been developed. However, the conventional polarization mode dispersion compensation methods can only compensate for a group delay time of a relatively small quantity, require expensive high-speed electronic parts, and have difficulties in scaling down in size. Further, the conventional methods have problems with respect to durability since they require moving parts.